In an air conditioning system with multiple outdoor units connected in parallel, various differences, such as a size of a heat exchanger of each outdoor unit, an inspiratory capacity of a compressor of each outdoor unit, using environment and system loading changed over time, as well as installing standardized degree, may lead to an uneven distribution of refrigeration returned from an indoor unit(s) between the outdoor units when the outdoor units are operated in a heating mode. Less refrigeration distributed to some outdoor units may be evaporated easily in the heat exchangers of these outdoor units and overheat may be formed; more refrigeration distributed to some outdoor units cannot be evaporated completely because heat exchange capacities of the heat exchangers of these outdoor units are limited. As a result, the overheat degrees of some compressors are too high, while the overheat degrees of some compressors are too low. The too high overheat degree of the compressor may lead to a poor heat dissipation of a motor of the compressor, and too high temperature may also result in a metamorphism of refrigerant oil in the compressor easily so as to lead a poor lubrication, thereby influencing the lifetime of the compressor; on the other hand, the too low overheat degree of the compressor may indicate that the refrigeration at an admission port of the compressor may be in a liquid state, and compressibility of the redundant refrigeration which has not been evaporated completely may be poor, which may lead to an increase of current power of the compressor, and the refrigerant oil may be diluted at the same time because of the liquid refrigeration, which may lead to a decrease of the refrigerant oil entered into the compression chamber of the compressor, so as to aggravate abrasion of the compression chamber. According to adjusting methods in the related arts, a compressor of a single outdoor unit is generally treated as an object to adjust. However, the adjusting of each outdoor unit in the air conditioning system may influence each other, so that the air conditioning system may not acquire an overall control. Thus, the overheat degrees of compressors in the air conditioning system must be guaranteed in a proper range, and there should not be great differences between the overheat degrees of the outdoor units.